Nanotube sensor

ABSTRACT

A sensor having a nanotube grown on and supported by thermal bimorph structures. The nanotube rests on a heat sink during sensing gas or a liquid and is moved from the heat sink when the nanotube is heated to desorb gas or liquid from it. The heatsink may function as a gate along with the bimorph structures as the other terminals of a transistor. Current-voltage and current-gate voltage characteristics may be obtained of the nanotube as a device like a transistor. These characteristics may provide information on a gas or liquid absorbed by the nanotube.

BACKGROUND

[0001] The invention pertains to gas sensors. Particularly, it pertains to nanotube sensors.

[0002] Certain attempts have been made to use nanotube for gas sensing. U.S. patent application Ser. No. 10/100,440, filed Mar. 18, 2002 and entitled, “Carbon Nanotube Sensor,” is hereby incorporated by reference.

SUMMARY

[0003] Nanotubes can be grown in-situ in a directed manner in a certain gas environment by bridging a moat between two moveable finger-or arm-like support pieces, connections or electrodes for the nanotubes. A manufacturable MEMS-base nanotube sensor can be made.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIGS. 1a and 1 b show a sensor structure that supports a nanotube.

[0005]FIGS. 2a and 2 b show a sensor structure that has a nanotube.

[0006]FIGS. 3a and 3 b show electrical configurations of a nanotube sensor.

[0007]FIG. 4 is a graph of current-gate voltage curves of several gases absorbed by a nanotube of a sensor.

[0008]FIG. 5 is a flow diagram of a process of a nanotube sensor.

[0009]FIGS. 6a and 6 b show a nanotube sensor having a cross-bar like structure.

[0010]FIGS. 7a and 7 b are cross-section views of a nanotube sensor similar to that in FIG. 6a.

[0011]FIGS. 8a and 8 b are cross-section views of a nanotube sensor similar to that in FIG. 6b.

DESCRIPTION

[0012]FIG. 1a shows a basic structure that supports a nanotube. Bimorph finger- or arm-like longitudinal structures 11 and 12 are situated on substrate 10. A pedestal-like structure 13 may be situated between the ends of structures 11 and 12 on substrate 10. Structure 13 may be utilized as a heat sink or an electrode or both.

[0013] Structures 11 and 12 may be composed of a top metal layer 21 and a bottom 22 dielectric, or vice versa. These structures may have other compositions. In FIG. 1b, structures 11 and 12 may be heated and their ends will move away from structure 13 and substrate 10 due to the heat.

[0014] In FIG. 2a, while fingers or arms 11 and 12 are raised up, one or more nanotubes may be grown bridging the two closer ends of structures 11 and 12. This growing may be performed in an atmosphere or local environment of ethylene, methane, CO or the like. A nanotube or nanotubes are grown upon heating the structures to between approximately 700 and 900 degrees Celsius. Generally, one may result, in having a carbon single-walled (i.e., one layer of molecules) nanotube 15 bridging the ends facing each other of structure 11 and 12. The nanotube may grow by virtue of flow and/or an electric field from one end to the other of structures 11 and 12. When device 14 is removed from the heat of a furnace or other heat source, structures 11 and 12 cool and their ends 16 and 17 move towards substrate 10 as shown in FIG. 2b. This activity causes nanotube 15 to sit on heat sink/electrode 13. Structure 13 may be composed of a metal 19 that has been coated with a thin passivation layer 18. Device 14 can be cut out as a die and mounted on a header for operation.

[0015] When nanotube 15 has cooled down, it may be exposed to a fluid and absorb some of it. A fluid may be a gas or a liquid. “Gas” is to be referred to in this description but may be interchanged with “liquid”. When device 14 is heated as a die, nanotube 14 is lifted off of post or pedestal 13 by structures 11 and 12 when the latter bend as a result of their bimorph structures being heated. When nanotube 15 is heated, absorbed gases are driven off nanotube 15, making it ready for resorption of new gases. When the heat is removed, arms 11 and 12 come down; nanotube 15 sits on heat sink 13 which removes heat from nanotube 15. Then nanotube 15 is ready for resorption of new gas in the sensor's immediate environment.

[0016] To lift nanotube 15 off of heat sink 13, arms 11 and 12 may be actuated several different ways. Device 14 as a die on a header 20 may be heated or structure 11 and 12 may have heating elements in them. In either case, nanotube 15 is removed from contact with heat sink 13 so that nanotube 15 can more rapidly heat up and desorb any gas on it, as shown in FIG. 2a. After this, heat is removed from header 20 or heating elements 23 in arms 11 and 12 are disconnected. Structures 11 and 12 move towards substrate 10 and nanotube 15 rests on heat sink 13 to further cool off and be ready for absorption of a new gas.

[0017]FIG. 3a shows an electrical configuration which may be used to aid in identifying a gas absorbed by nanotube 15. Power supply 24 is connected to structure 11 and 12 which become connections to nanotube 15. The current indication from meter 26 and the voltage indication from meter 25 may be noted, and from such IV characteristics, information about or identification of a gas or liquid absorbed by nanotube 15 may be obtained.

[0018]FIG. 3b shows a three terminal electrical configuration resembling a field effect transistor having a power supply 27 and a meter 28 for measuring gate-voltage. Structure 13 is the gate and structures 11 and 12 are the source and drain, respectively. The other electrical aspects of this figure are similar to those shown in FIG. 3a. The current (I) meter 26 and the gate voltage (V_(g)) meter 28 may be noted, and from such IV_(g) characteristics, information about or identification of a gas or a liquid absorbed by nanotube 15 may be obtained. On the other hand, for a predetermined V_(g) set by power supply 27, the current from meter 26 and the voltage from meter 25 may be noted, and from such IV characteristics, information about or identification of a gas or liquid absorbed by nanotube 15 may be obtained. The above readings may be taken periodically over a period of time from the moment that a gas begins to be absorbed by nanotube 15. Such IV and IV_(g) characteristics may provide additional information, such as concentrations of the gas or liquid in the environment of nanotube 15.

[0019]FIG. 4 is an example of IV_(g) characteristics for several gases absorbed by device 14 in FIG. 3b. Curve 31 shows the IV_(g) characteristics before gas absorption by nanotube 15. Curve 32 shows that IV_(g) characteristics after absorption of NH₃ by nanotube 15. Curve 33 shows the IV_(g) characteristics after absorption of NO₂ by nanotube 15. Absorption of each gas would occur after any gas or liquid in nanotube 15 was desorbed or removed. The nanotubes could be functionalized with different materials (metals, organics, semiconductors) to enhance response and discrimination for different gases.

[0020]FIG. 5 shows a flow diagram 30 outlining the gas/liquid sensing process of device 14 as described above. Block 34 is exposure of nanotube 15 to the gas and/or liquid. Measuring the IV and/or the IV_(g) characteristics of nanotube 15 is represented by block 35. Next, in block 36, nanotube 15 is moved away from heat sink 13 by support structures 11 and 12 with heat.

[0021] Heat is also used to drive off gas/liquid from nanotube 15, as represented by block 37. In block 38, structures 11 and 12 are cooled and return nanotube 15 to rest on heat sink 13 to further cool. Path 39 shows that the sensing process may repeat for sensing another or the same gas or liquid.

[0022]FIG. 6a reveals a plan view of a sensor 41 having a structure or crossbar 40 that may be a heat sink and/or an electrode of a transistor for purposes of attaining IV_(g) characteristics of nanotube 15, particularly for gas/liquid sensing. Structures 47 and 48 may be the other connections to the transistor, i.e., nanotube 15. FIGS. 7a and 7 b show a cross-section view of device 41. Nanotube 15 is situated between crossbar 40 and substrate 43. Structures 47 and 48 may have heating elements to heat them and nanotube 15, or substrate 43 may be heated for a similar effect. Upon heating, structures 47 and 48 lower nanotube 15 off of heatsink 40. Upon cooling, nanotube 15 is brought up to heatsink 40 and nanotube 15 is further cooled by structure 40. There may be an inverted pyramid-shaped pit 44 etched in substrate 43, for cooling or other reasons. Some other aspects of device 41 for gas/liquid sensing are like that of device 14.

[0023]FIG. 6b shows a device 42 which has a crossbar, heatsink or gate-like structure 40 situated between nanotube 15 and substrate 43. Along with structure 40, structures 45 and 46 which are connected to nanotube 15, and with configurations like those of device 14 in FIGS. 3a and 3 b, one may get IV and IV_(g) data of nanotube 15 with or without an absorbed gas or liquid. FIGS. 8a and 8 b show a cross-section of device 42. Finger- or arm-like longitudinal structures 45 and 46 that hold nanotube 15 are like that of structures 47 and 48 of device 41 except that they move away from substrate 43 rather than towards it, when heated. When structures 47 and 48 cool down, they move towards substrate 43 and nanotube 15 may be set on heatsink structure 40 for further cooling. There may be a pyramid-shaped pit 44 in substrate 43 to possibly improve cooling or facilitate other reasons for device 42. Or there may not be a pit 44. Many aspects of device 42 are like those of devices 14 and 41 for gas/liquid sensing.

[0024] Longitudinal structures 45, 46, 47 and 48 may be etched, at least in part, from substrate 43, or be formed on substrate 43. Structure 40 may be made in a similar fashion like that of structures 45, 46, 47 and 48. The structures of devices 14, 41 and 42 may be MEMS technology or be compatible with it. The technology of these devices may be silicon based or of another material.

[0025] Although the invention has been described with respect to at least one specific embodiment, many variations(and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. 

What is claimed is:
 1. A nanotube sensor comprising: a first structure; a second structure having a first end attached to said first structure, and having a second end; a third structure having a first end attached to said first structure, and having a second end; and at least one nanotube attached to the second ends of said second and third structures.
 2. The sensor of claim 1, further comprising a fourth structure situated on said first structure and proximate to said at least one nanotube.
 3. The sensor of claim 2, wherein the second ends of said second and third structures may move relative to said first structure upon a change of temperature.
 4. The sensor of claim 3, wherein said fourth structure may absorb heat.
 5. The sensor of claim 4, wherein said at least one nanotube may be close to said fourth structure.
 6. The sensor of claim 5, wherein a change of temperature of said second and third structures may cause said at least one nanotube to be moved away from or closer to said fourth structure.
 7. The sensor of claim 6, wherein a current-voltage (IV) characteristic of said at least one nanotube is measurable.
 8. The sensor of claim 7, wherein said at least one nanotube is exposable to a fluid.
 9. The sensor of claim 8, wherein the fluid may be a gas and/or liquid.
 10. The sensor of claim 9, wherein the IV characteristic of said at least one nanotube may indicate to what kind of fluid said at least one nanotube is exposed.
 11. The sensor of claim 10, wherein heating said at least one nanotube may remove a significant portion of a fluid gas to which said at least one nanotube was exposed.
 12. The sensor of claim 11, wherein the heating may result in said second and third structures to move said at least one nanotube away from said fourth structure.
 13. The sensor of claim 12, wherein a movement of said at least one nanotube away from said fourth structure may result in increased heating of said at least one nanotube.
 14. The sensor of claim 13, wherein cooling may result in said second and third structures to move said at least one nanotube closer to said fourth structure.
 15. The sensor of claim 14, wherein a movement of said at least one nanotube closer to said fourth structure may result in increased cooling of said at least one nanotube.
 16. The sensor of claim 15, wherein said at least one nanotube may be heated by said second and third structures.
 17. The sensor of claim 16, wherein said second and/or third structures have a bimorph composition.
 18. The sensor of claim 15, wherein said at least one nanotube is heated by said first structure.
 19. The sensor of claim 18, wherein said second and/or third structures have a bimorph composition.
 20. The sensor of claim 6, wherein: said sensor is like a transistor; and said second, third and fourth structures are a source, a drain and a gate of the transistor, respectively.
 21. The sensor of claim 20, wherein a current-gate voltage (IV_(g)) characteristic of the sensor is measurable.
 22. The sensor of claim 21, wherein said at least one nanotube is exposable to a fluid.
 23. The sensor of claim 22, wherein the fluid may be a gas or liquid.
 24. The sensor of claim 23, wherein the IV_(g) characteristic may indicate to what kind of fluid said at least one nanotube is exposed.
 25. The sensor of claim 24, wherein heating said at least one nanotube may remove a significant portion of a fluid to which said at least one nanotube was exposed.
 26. The sensor of claim 25, wherein the heating may result in said second and third structures moving said at least one nanotube away from said fourth structure so as to increase the temperature of said at least one nanotube.
 27. The sensor of claim 26, wherein the cooling may result in said second and third structures moving said at last one nanotube closer to said fourth structure so as to decrease the temperature of said at least one nanotube.
 28. The sensor of claim 27, wherein at least one of said second and third structures has a thermal bimorph composition.
 29. The sensor of claim 28, wherein at least one of said first, second, third and fourth structures is made with a MEMS based process.
 30. The sensor of claim 2, wherein said at least one nanotube is situated between said first structure and said fourth structure.
 31. The sensor of claim 30, wherein said fourth structure has first and second ends attached to said first structure.
 32. The sensor of claim 31, wherein a current-voltage (IV) characteristic of said at least one nanotube is measurable.
 33. The sensor of claim 32, wherein: said at least one nanotube is exposable to a fluid; and the fluid may be a gas and/or liquid.
 34. The sensor of claim 33, wherein the IV characteristic of said at least one nanotube may indicate to what kind of fluid said at least one nanotube is exposed.
 35. The sensor of claim 31, wherein: said sensor is like a transistor; and said second, third and fourth structures are like a source, a drain and a gate of transistor, respectively.
 36. The sensor or claim 35, wherein a current-gate voltage (IV_(g)) characteristic of the sensor is measurable.
 37. The sensor of claim 36, wherein; said at last one nanotube is exposable to a fluid; and the fluid may be a gas and/or liquid.
 38. The sensor of claim 37, wherein the IV_(g) characteristic may indicate to what kind of fluid said at least one nanotube is exposed.
 39. The sensor of claim 38, wherein: heating said at least one nanotube may remove a significant portion of a fluid to which said at least one nanotube was exposed and absorbed; and cooling said at least one nanotube may enable reabsorptance of a fluid.
 40. The sensor of claim 39, wherein said second and third structures upon heating move said at least one nanotube away from said fourth structure and upon cooling move said at least one nanotube towards said fourth structure.
 41. The sensor of claim 40, wherein said second and third structures have a temperature bimorph structure.
 42. The sensor of claim 39, wherein said fourth structure is a longitudinal structure situated approximately non-parallel to said at least one nanotube.
 43. The sensor of claim 42, wherein said first structure has a pit proximate to said at least one nanotube.
 44. The sensor of claim 2, wherein said at least one nanotube is situated between said fourth structure and said first structure; and said fourth structure has first and second ends attached to said first structure.
 45. The sensor of claim 44, wherein: a current-voltage (IV) characteristic of said at least one nanotube is measurable; said at least one nanotube is exposable to a fluid; the fluid may be a gas and/or liquid; and the IV characteristic of said at least one nanotube may indicate to what kind of fluid said at least one nanotube is exposed.
 46. The sensor of claim 45, wherein: said sensor is like a transistor; said second, third and fourth structures are like a source, a drain and a gate of the transistor, respectively; a current-gate voltage (IV_(g)) characteristic of the sensor may be measurable; said at least one nanotube may be exposable to a fluid; the fluid is a gas and/or liquid; and the IV_(g) characteristic may indicate to what kind of fluid said at least one nanotube is exposed.
 47. The sensor of claim 46, wherein said fourth structure is a longitudinal structure situated approximately non-parallel to said at least one nanotube.
 48. The sensor of claim 47, wherein said first structure has a pit proximate to said at least one nanotube.
 49. The sensor of claim 47, wherein said second and third structures upon heating move said at least one nanotube away from said fourth structure and upon cooling move said at least one nanotube towards said fourth structure.
 50. The sensor of claim 49, wherein the sensor is made with a MEMS-based process.
 51. A nanotube sensor comprising: a substrate; a first longitudinal projection on said substrate; a second longitudinal projection on said substrate; and at least one nanotube grown and connecting first and second longitudinal projections.
 52. The sensor of claim 51, wherein said first and second projections, when heated, move away from said substrate, and when cooled, move towards said substrate.
 53. The sensor of claim 52, wherein: said projections have an ambient environment of ethylene, methane, CO or other like fluid; the environment is heated to a temperature sufficient to move said projections away from said substrate; the temperature is sufficient to grow said at least one nanotube; and after the said at least one nanotube is grown, the environment cools down and said projections, along with said at least one nanotube, move towards said substrate.
 54. The sensor of claim 51, further comprising: a heat sink structure which said at least one nanotube can be situated upon a moving of said first and second projections towards said substrate; and wherein: said at least one nanotube can absorb a gas/liquid in a proximate environment; and current-voltage (IV) characteristics of said at least one nanotube may reveal information about the gas absorbed.
 55. The sensor of claim 54, wherein: upon heating said first and second projections, said at least one nanotube is moved away from said heat sink structure and heated thereby desorbing a significant portion of gas/liquid on said at least one nanotube; and upon cooling said first and second projections, said at least one nanotube may be moved to said heat sin structure to cool and may absorb some gas/liquid.
 56. The sensor of claim 55, wherein: said sensor is like a transistor in that said first projection, said second projection and said heat sink structure are like source, drain and gate connections of the transistor; and current-gate voltage (IV) characteristics of said at least one nanotube may reveal real information about a gas absorbed by said at least one nanotube.
 57. The sensor of claim 51, wherein said first and second projections move in a first direction when heated and move in a second direction when cooled.
 58. The sensor of claim 57, further comprising a longitudinal strip having first and second ends attached to said substrate.
 59. The sensor of claim 58, wherein: said at least one nanotube is situated between said longitudinal strip and said substrate; and said first and second projections are like a source and a drain, respectively, and said longitudinal strip is like a gate of a transistor.
 60. The sensor of claim 59, wherein a current-voltage characteristic of said at least one nanotube may indicate information about a gas absorbed by said at least one nanotube.
 61. The sensor of claim 59, wherein a current-gate voltage characteristic of said sensor may indicate information about a gas absorbed by said at least one nanotube.
 62. The sensor of claim 58, wherein: said longitudinal strip is situated between said at least one nanotube and said substrate; and said first and second projections are like a source and a drain, respectively, and said longitudinal strip is like a gate of a transistor.
 63. The sensor of claim 62, wherein a current-voltage characteristic of said at least one nanotube may indicate information about a gas absorbed by said at least one nanotube.
 64. The sensor of claim 62, wherein a current-gate voltage characteristic of said sensor may indicate information about a gas absorbed by said at least one nanotube.
 65. A nanotube sensor comprising: means for removing heat; and means for supporting and moving a nanotube relative to said means for removing heat; and wherein said means for supporting and moving the nanotube moves the nanotube away from said means for removing heat when being heated and moves the nanotube closer to said means for removing heat when being cooled.
 66. The sensor or claim 65, further comprising means for measuring the current-voltage characteristic of nanotube, wherein said means for measuring is connected to the nanotube.
 67. The sensor of claim 66, wherein a current-voltage characteristic of the nanotube may indicate information about a gas or liquid absorbed by the nanotube.
 68. The sensor of claim 67, further comprising a means for heating the nanotube.
 69. The sensor of claim 65, further comprising: means for providing a gate proximate to the nanotube; and means for measuring the current-gate voltage characteristic of the nanotube; and wherein: a first connection and second connection to the nanotube are a source and a drain, respectively; and together with said means for a gate, the source and drain result in the nanotube being like a transistor.
 70. The sensor of claim 69, wherein the current-gate voltage characteristic of the nanotube may indicate information about a gas or liquid absorbed by the nanotube.
 71. The sensor of claim 70, further comprising a means for heating the nanotube.
 72. A method for sensing a gas/liquid, comprising: obtaining a structure having first and second terminals connected to the ends of a nanotube, respectively; exposing the nanotube to a gas/liquid; measuring a current-voltage characteristic of the nanotube; and reviewing the current-voltage characteristic to obtain possible information about the gas/liquid.
 73. The method of claim 72, further comprising: heating the nanotube to desorb the gas/liquid from the nanotube; and removing heat from the nanotube to prepare the nanotube to absorb gas/liquid.
 74. A method for sensing a gas or liquid, comprising: obtaining a structure having source and drain terminals connected to a nanotube, and a gate terminal proximate to the nanotube; exposing the nanotube to a gas or liquid; measuring a current-gate voltage characteristic of the nanotube; and reviewing the current-gate voltage characteristic to obtain possible information about the gas or liquid.
 75. The method of claim 74, further comprising: heating the nanotube to desorb the gas or liquid from the nanotube; and removing heat from the nanotube to prepare the nanotube to absorb gas or liquid. 